The present invention relates to a method and apparatus for minimizing the effects of pellet-cladding interaction (PCI) in nuclear fuel rods. The present invention also relates to coated nuclear fuel pellets and more particularly to fuel pellets coated with a burnable poison.
It is known that nuclear fuel may have various shapes such as plates, columns, or fuel rods comprising fuel pellets disposed in end-to-end abutment within a tube or cladding made of a zirconium alloy or stainless steel. Such fuel pellets contain fissionable material, such as uranium dioxide, plutonium dioxide, or mixtures thereof. The fuel rods are usually grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.
The nuclear fuel pellets in a fuel rod may interact with the tube or cladding in an undesirable manner. Theoretically, the PCI phenomenon may even result in cladding fracture whereby the fuel pellets are exposed to reactor coolant water resulting in the introduction of radioactive fission products into the coolant. One method of controlling PCI is to position cushioning layers of non-fuel material between the fuel and the cladding. Such layers may be attached to either the fuel outside diameter or to the cladding inside diameter. Either is sufficient as long as the layer prevents direct contact between the fuel pellets and the cladding.
U.S. Pat. No. 3,427,222 describes coating of burnable poisons applied to fuel pellets of the above-described configuration. One of the preferred designs of this patent is a coating of pure zirconium diboride (ZrB.sub.2) applied to the fuel pellets as a layer of about 5 to 10 microns (0.2 to 0.4 mils) thick although coatings in the range of 0.02 to 5.0 mils are discussed.
However, to achieve the proper separation of the fuel and cladding and to prevent undesirable PCI, a layer of 10 to 100 microns (0.4 to 4 mils) thick is desirable. Such relatively thick layers should be good conductors of heat, such as zirconium diboride, so as not to unduly interfere with heat exchange between the fuel pellets and the coolant.
Unfortunately, if zirconium diboride or other known burnable poisons were added to all of the fuel pellets in a reactor with no regard to the amount of boron-10 (an isotope constituent of natural boron) or other neutron absorbing isotopes present, and in thicknesses of 10 to 100 microns, the reactor core would contain too much poison to operate. In other words, the burnable poisons would more than compensate for the excess reactivity of the core. As used herein, burnable poisons comprise neutron absorbing poisons which burn faster than the nuclear fuel.
A reactor core is typically operated to produce heat which is converted to steam. The steam may then be used to produce electricity or for other purposes. When a new reactor starts, its core is often divided into a plurality, e.g. three, groups of assemblies which may or may not be distinguished by their position in the core but which are usually distinguished by the enrichment of the nuclear fuel in the fuel pellets. For example, a first batch or region may be enriched to an isotopic content of 2.0% uranium-235. A second batch or region may be enriched to 2.5% uranium-235 and a third batch may be enriched to 3.5% uranium-235. After ten to eighteen months of operation, the reactor would typically be shut down and the first batch would be removed from and replaced by a new batch, enriched to about 3.5% uranium-235. Subsequent cycles would repeat this sequence at intervals in the range of from about eight to eleven months. Refueling as described above is required since the reactor operates as a nuclear device to produce heat only as long as it remains a critical mass. In order for a core to remain a critical mass at the end of a given period of operation, it must possess excess reactivity, k, at the beginning of operation.
Power reactors are typically provided with sufficient excess reactivity at the beginning of a cycle to allow operation for a period of about six to eighteen months. Since a reactor operates only slightly supercritical, the excess reactivity supplied at the beginning of a cycle must be counteracted. It is for this reason that various schemes such as partial insertion of control rods or adding neutron absorbing poisons to the core or fuel are used. Combinations of the above-described control methods may be employed to improve the efficiency of control of excess reactivity as evident by U.S. Pat. No. 3,349,152; U.S. Pat. No. 3,372,213, or U.S. Pat. No. 3,427,222; and EPRI Report NP-1974.
The use of control rods to control excess reactivity introduces a disadvantage in that it effectively removes part of the active core. This removes part of the moderator as well and makes the reactor less efficient in its creation and use of neutrons.
The use of neutron absorbing poisons in the coolant is better in terms of efficiency but is limited by other considerations. For example, a boiling water reactor can use practically no water soluble salt with poisons in its coolant since these salts would be left on the fuel rods and interfere with heat transfer and would also accelerate corrosion as the coolant water evaporated to form steam.
The amount of burnable poison used in pressurized water reactor coolant (chemical shim) is limited by the fact that as the reactor heats up, some of the coolant is forced out of the core by thermal expansion. Since the coolant is both a moderator and the poison, the reactivity of the core will not progressively increase as the reactor heats up unless the coolant is more poison than moderator. This condition of progressive increase in reactivity during heat up by forcing soluble poison out of the core with moderator water occurs when the coolant contains more than about 1200 ppm of natural boron in solution.
While boron may be used as chemical shim to counteract excess reactivity for a period of five to six months, for the reasons stated above, it cannot be used as a chemical shim if a longer cycle is desired. Under those circumstances either control rods must be used or some other form of burnable poison supplied.
Incorporation of burnable poison in fuel assemblies has been recognized in the nuclear field as an effective means of increasing fuel capacity and thereby extending core life. Burnable poisons are used either uniformly with the fuel (i.e. distributed poison) or placed discretely on separate elements in the reactor, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core. U.S. Pat. No. 3,427,222.
It is known that nuclear fuel contained in an aluminum can may be coated with a layer of niobium to prevent the fuel from reacting with the can (U.K. Pat. No. 859,206). It is also known that minute nuclear fuel particles, such as uranium dioxide particles may be coated with a single layer or several layers of the same or different non-poison materials, including niobium for such purposes as protecting the fuel from corrosion and helping to retain the products of fission. The coating may be applied by various techniques such as depositing from a vapor of the coating material, depositing from a decomposing vapor, and electroplating (U.K. Pat. No. 933,500).
Japanese Pat. No. 52-3999 describes a nuclear fuel which is first coated with a thin layer of material such as niobium to absorb fission fragments and then coated with a main coating material such as zircoloy. This reference is not concerned with burnable poison coatings.
In Dispersion Fuel Elements, an AEC monograph by A. N. Holden published in 1967 by Gordon and Breach of New York, there is described coating fuel particles in dispersion fuels to prevent chemical interaction of the particles with the matrix and to retain fission products (page 30). Uranium dioxide coated with niobium by vapor phase reduction is also discussed (page 48). Also discussed is uranium dioxide coated with chromium, by vapor phase reduction using chromium dichloride, which was deposited over a niobium undercoat (page 48).
Accordingly, it will be appreciated that there remains an unsolved need for a fuel assembly design wherein a protective layer is provided between substantially all of the fuel pellets and the cladding in the core and at the same time introducing an appropriate amount of burnable poison into the core to control excess reactivity and enhance operating efficiency.